Files created offline, for instance form Word (.doc) converted to PDF or XML, are not uploadable.

Not sure if the PDF file was generated online?

Right click on the file icon, select Properties:

Click on tab PDF and check the PDF producer label:

If the name of the PDF producer is not LibreOffice and was modified using a different converter than the iText (for example LibreOffice 4.0, modified using iText 5.5.1 as shown above), the file cannot be uploaded.